The invention relates generally to imaging lidar (light detection and ranging) systems. More particularly, this invention relates to a method and apparatus for three dimensional imaging.
There is a continuing need to develop methods of detecting underwater targets from remote locations (e.g. airborne) and over relatively short time periods. This is of particular importance in the case of certain military applications where, for example, the detection of moored mines from helicopters and other aircraft is vital to ensuring safe seas. Presently, cumbersome and time consuming wire line devices must be used. These devices are lowered into the water and of course, are easily subject to damage and loss. Also, wire line devices make target searching relatively slow and can only detect targets without providing visual imaging for classification and identification.
In addition to detection of underwater mines, other military applications for underwater target detection systems include the detection of submarines and communication buoys. There are also a number of civilian applications for an accurate system for remote detection of underwater objects.
The prior art has addressed this problem with the imaging lidar system of U.S. Pat. No. 4,862,257 which is assigned to assignee hereof and incorporated in its entirety herein by reference. In accordance with the lidar system of U.S. Pat. No. 4,862,257, a laser is used to generate short pulses of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water and to an object or target. Intensified CCD (charge coupled device) cameras (preferably two or more)are electronically shuttered after a time delay corresponding to the round trip propagation time to and from the target. This timing eliminates light scattered by the water from in front of and behind the target As a result, the veiling luminance of the water is greatly attenuated and faint target signatures can be seen. The resulting gated images (displayed on a CRT) have sufficient spatial resolution to classify and/or identify the target. This imaging feature offers the potential of reduced false alarm rates compared to non-imaging systems.
This imaging lidar system takes advantage of the fact that all opaque targets will cast a shadow. The exposure of the two cameras in the system are timed such that one camera gives an image of the target against the backscattered light from the water while the other camera gives an image of the shadow of the target against the backscattered light. These two images are then subtracted (e.g., differential imaging) to improve the detectability of the target. The subtracted image can then be colorized to assist in identification of the target.
In effect, the subtraction of the two images provides improved signal-to-noise ratio (SNR) of the imaged target. The SNR provided by the differential imaging is predicted by a theoretical model which is given by the detected target photons minus the detected water background photons over the square root of the sum of the detected photons from the target and twice the number of photons from the background.
Other examples of imaging lidar systems are disclosed in commonly assigned U.S. Pat. Nos. 4,964,721; 4,967,270; 5,013,917 and 5,091,778 all of which are incorporated by reference herein in their entirety.
The above prior art systems require the use of multi-gated two-dimensional images to obtain desired performance. In these prior art systems, sizes of small imaging arrays are limited by the maximum aperture of the collection optics which makes it necessary to use many sensors to obtain a large collection aperture and/or many time-resolved images. Accordingly, a need exists for an improved imaging system which is more compact and well suited for efficient light weight operation (e.g., aircraft on board imaging systems).